Fixed workflow for django 4

dav_events/workflow.py

@@ -264,6 +264,9 @@ class BasicWorkflow(object):
         status_list = []
         self._check_status()
 
+        if not event.id:
+            return status_list
+
         heaviest_flag = event.flags.order_by('status').last()
         if heaviest_flag:
             flags = []